The present invention relates to a low-pass filter for a wireless communication system; and, more particularly, to a HTS low-pass filter for suppressing broadband harmonics.
Recently, as various wireless communication systems and services are developed intensively, the considerable performance improvement such as small insertion loss, high selectivity, high sensitivity and small size are needed in development of communication components for a cellular phone and a personal communication system. In order to satisfy these demands, the development of materials, design (circuits) and fabrication (processes) technologies are essential for the communication devices.
Since low-pass filter (LPF) is a frequency selective and passive device with low levels of attenuation, LPF is widely used to reject harmonics or spurious signals in a integrated mixer, a voltage controlled oscillator (VCO) and so on. But an open-stub type low-pass filter and a step-impedance type low pass filter have a narrow stopband (about 3 times of cutoff frequency in case of a conventional LPF).
FIGS. 1A and 1B show an equivalent circuit diagram and a schematic diagram of a conventional microstrip low-pass filter.
FIG. 1A shows the equivalent circuit diagram of the lumped-element low-pass filter designed through the transformation of impedance level and frequency scale from the prototype low-pass filter (not shown). The lumped-element low-pass filter (or xcfx80-type low-pass filter) includes an inductance L2 corresponded to the microstrip transmission line, a first shunt capacitance C1 and a second shunt capacitance C2 corresponded to the two parallel microstrip open-stubs (in this case: C1=C2).
Referring to FIG. 1B, the conventional microstrip low-pass filter includes a crystalline substrate 180 (hereinafter, referred to as xe2x80x9can MgO substratexe2x80x9d), a signal transmission input port 150 and a signal transmission output port 160, two parallel stripe lines 170 of a microstrip open-stub type, a microstrip line 140 and a ground plane 190.
The signal transmission input port 150 and the signal transmission output port 160 are fabricated on both edges of the top face of the MgO substrate 180. Two parallel microstrip open-stubs 170 between the signal transmission input port 150 and the signal transmission output port 160 are perpendicular to a signal propagation direction. Therefore, the microstrip line 140 is perpendicular to two parallel microstrip open-subs 170. The groundplane (e.g., Au or Ag film) 190 is coated at the bottom (backside) of the MgO substrate 180.
In general, there are some problems in the conventional low-pass filter as described above. Since the conventional low-pass filter has a narrow stopband range in frequency domain, an interference occurred by the adjacent wireless communication systems and a noise generated by the communication system itself are quite serious.
It is, therefore, an object of the present invention to provide a low-pass filter having a high-efficiency broad stopband characteristics, in which attenuation poles and a frequency range of the stopband can be controlled easily.
In accordance with an aspect of the present invention, there is provided a low-pass filter comprising: a circuit pattern having at least one or more units, wherein the circuit pattern includes a coupled line section having a pair of parallel stripe lines and a transmission line section having a pair of parallel straight lines whose two ports of one side are opened and whose two ports of the other side are connected to each other, each port of one side of the pair of the parallel straight lines being connected with each port of one side of the coupled line section.